Why the need for magenta?
Magenta has beeen tested and proven to be more intense over a wider range of conditions, but as you said, people already ignore ambers.
I used to have my rear flashing strobes painted red (and the local council has red strobes on some of their vehicles) , and this is something that is/has/was looked into as well, as people tend to slow down for red, but the ACPO had reservations over vehicles being driven with these lights on, they wanted them working only when stationary. This was about 2002, so i think that was long forgotten.